Preheat the oven to 400 degrees F (200 degrees C)
Heat butter in a large deep skillet over medium heat. Add onion and celery and cook until tender, about 4 minutes
Sprinkle onion mixture with salt, pepper, and garlic powder and cook for 1 minute
Add flour and cook, stirring constantly, about 1 minute
Whisk in broth and cream and bring mixture to a simmer, stirring often
Cook, stirring occasionally, about 5 minutes
Mix in chicken and vegetables and cook 2 minutes
Adjust seasoning if necessary and remove from heat
Lightly grease a rimmed baking sheet and pour chicken mixture into the prepared baking sheet
Lightly roll pastry sheets to fit the baking sheet if needed and lay pastry on top to cover chicken mixture
Bake in the preheated oven until golden brown and bubbly, 25 to 30 minutes
